1. The problem asks about the change in allele frequencies due to migration of a small subgroup of a population. 2. This is a concept in population genetics where a small group separates from a larger population and establishes a new population. 3. The key term here is the "founder effect," which occurs when a new population is started by a small number of individuals, carrying only a fraction of the genetic variation of the original population. 4. This effect can cause allele frequencies in the new population to differ significantly from the original population due to the limited genetic diversity of the founders. 5. Other options like the bottleneck effect involve a drastic reduction in population size, natural selection involves differential survival and reproduction, and genetic drift is random changes in allele frequencies but not specifically due to migration of a subgroup. 6. Therefore, the correct answer is **The founder effect**.